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One in a hundred American children has an autism spectrum disorder (ASD). That stunning new statistic was released on Monday by the Federal Government, officially revising the 2007 federal estimate of 1 in 150 children. The new number puts U.S. prevalence on par with reported rates in England, Japan, Sweden and Canada. It is based on two separate and very different government-funded research studies: a telephone survey of 78,037 parents by the Health Resources and Services Administration (HRSA), and a rigorous national surveillance study conducted by the Centers for Disease Control and Prevention (CDC). In an unusual show of attention and concern, top officials from the Department of Health and Human Services, the National Institutes of Health and the CDC held a press conference on Oct. 2 in which they attempted to explain the new numbers, allay concerns and assure the public that substantial government resources are being devoted to understanding autism. (See pictures of the world of autism.)

"We are extremely concerned about the apparent increase in estimated prevalence of autism spectrum disorders," said Ileana Arias of the CDC. "However, we urge extreme caution in interpreting this change. An increase in diagnosis does not necessarily mean that more children actually have ASD. Unfortunately, the information that we currently have doesn't allow us to give a true account of whether the apparent increase is an actual increase or the result of changes in the way we describe and diagnose ASDs."

Indeed, there is considerable uncertainty in the scientific and public-health communities about the meaning of the dramatic rise in autism numbers. Several factors other than a true increase in autism incidence have contributed to the ballooning numbers. These include greater awareness on the part of parents, pediatricians and educators; much broader definitions of autism than in decades past, when only the most severe form of the disorder was recognized (today, ASD includes the milder forms known as Asperger's syndrome and pervasive developmental disorder, not otherwise specified); earlier diagnosis of ASD, which can now be recognized by age 2 in many cases; and the growing availability of special services and interventions for children identified with ASD. (See pictures of inside a school for autistic children.)

It is not clear, however, that these factors can entirely account for the dramatic rise in autism numbers. "I don't think we can explain away all of the increase with these artifacts," said Dr. Thomas Insel, director of the National Institute of Mental Health, in discussing the new studies at Friday's press conference. "Buried in these numbers there may be a true increase."

The flurry of federal attention was occasioned by the publication of the HRSA survey in the journal Pediatrics. The survey  part of the 2007 National Survey of Children's Health  contained a number of intriguing data points. Most notable was the surprisingly high prevalence rate: one in 91 children ages 3 to 17 (1.1%) were described by parents as having an ASD diagnosis. Among boys, who are four times as likely to have autism as girls, the rate was 1 in 58. Even more mysterious, an additional group of children  0.6% of the sample  were described by parents as having had an ASD diagnosis in the past, but suffered from it no longer.

HRSA researcher Michael Kogan, the lead author of the study, admitted that it was hard to explain these vanishing cases of autism  which is by definition a lifelong condition. Yet nearly 40% of children who were diagnosed with autism, according to parents, no longer had the condition. It may be that such children received the autism label "to facilitate services for other conditions such as developmental delays," Kogan said. Or it could be that children were only tentatively classified as having ASD when they were very young and then the disorder was ruled out. (Read "Inside the Autistic Mind.")

The parent survey indicated that autism is more commonly diagnosed among white non-Hispanics than other groups. African-American children were 57% less likely to be diagnosed with an ASD than whites; they were also more likely to be in the group that "lost" the diagnosis. Mild autism was the most common type reported by parents. Half of parents said their child had a mild form of ASD, one-third described the child's condition as "moderate" and the remaining 17% said their child was severely affected. Parents also indicated that nearly 9 out of 10 (87%) children with ASD also had other mental-health issues such as attention deficit disorder and anxiety.

Experts not involved in the study caution that parent surveys are not the gold standard for measuring the prevalence of a medical condition. "The fact that 40% of the parents reporting that their child had received an ASD diagnosis now say the child no longer met criteria does suggest that there may be over-reporting in this survey," says Craig Newschaffer, a leading autism epidemiologist at Drexel University School of Public Health. "Nonetheless, the survey reinforces what we have come to understand over the past decade  that autism is much more common than previously thought." (See six tips for traveling with an autistic child.)

What's significant is that the study lines up well with other, more rigorous studies finding a 1% rate of autism. "It provides what scientists call convergent validity: no matter how you shake the bushes, you come up with this 1%," says Richard Roy Grinker, an autism researcher at George Washington University who has worked to determine ASD prevalence in South Korea.

The CDC's 1% figure is based on a more substantive study design than parent interviews. The CDC uses a network of up to 11 sites around the country known as the Autism and Developmental Disability Monitoring network (ADDM) to gather medical and special education records on 8-year-olds. Researchers and clinicians actually go through the records in an attempt to confirm diagnoses and identify children who may have been missed. Details of the CDC study are not expected to be released until December.

Autism advocacy groups responded to the new federal numbers by pressing for a greater federal commitment to autism research  particularly on possible environmental causes  and to treatment. "It is imperative that more resources be given to autism research so we can understand the causes and biology of autism and develop more effective treatments," said Geraldine Dawson, chief scientific officer for Autism Speaks, the largest ASD advocacy group in the U.S.

Anticipating this concern, federal officials took pains to emphasize that federal dollars devoted to autism have been rising in recent years. According to Peter Van Dyke, who oversees HRSA's Maternal and Child Health Bureau, President Obama has proposed an increase from $42 million this year to $48 million in 2010 for HRSA's autism-related programs. Insel, who heads the federal Interagency Autism Coordinating Committee, noted that autism is the only disorder specifically targeted for federal stimulus funds, to the tune of $85 million over the next two years.